As to world war I fighter pilots using thermals, I believe that the noise of an engine tends to overwhelm the sensitivity of the backside; it must have been very difficult to believe that a heavier than air craft could actually ascend in thermal, despite the evidence of buzzards, seagulls, and bits of paper....
Nympsfield is indeed an interesting hilltop site. Challenging in lots of ways, that dip in the middle, those wind turbines, etc. Observing a competition finish there one day, I saw the glider execute the customary low pass over the Committee, dumping his water ballast on the dignitaries, and then get caught in the curlover on the downwind leg. To everyone's delight, he landed out in the valley. Oh I say, good finish!